1. Field of the Invention
The present invention relates to an electrode material for an electrolytic capacitor, and more particularly, to an electrode material for an electrolytic capacitor having unprecedentedly high capacitance properties.
2. Description of the Related Art
Recent smaller and higher-reliability electronic devices strongly demands the smaller size and the higher capacity of an electrolytic capacitor.
An electrolytic capacitor typically has such a structure in which a capacitor element is formed by am anode electrode foil and a cathode electrode foil, wound together with an intervening separator made of manila paper and the like; the anode electrode foil is made of a band-shaped high purity aluminum foil, wherein the effective aluminum foil surface is enlarged through a chemical or electrochemical etching process, and wherein an oxide film is formed on the surface through a chemical process of treating the aluminum foil with chemical solution such as adipic acid ammonium solution and the like; while the cathode electrode foil is made of a high-purity aluminum foil that is subjected only to an etching treatment. Next, the capacitor element, after impregnating with electrolyte solution for driving the electrolytic capacitor, is housed into a bottomed outer case comprising aluminum. The outer case is equipped at the opening with a sealing member comprising an elastic rubber, and is sealed by drawing.
Development of an etching technique is conducted with respect to this type of an aluminum electrolytic capacitor, wherein, aiming to enhance the capacitance thereof, the effective surface area of an etching foil is extended so that the capacitance per unit area will be improved, whereby the effective surface area of the etching foil is extended. As part of such an etching technique, development of etchant composition and of current waveform to be applied at the time of etching is conducted. (Patent Documents 1 and 2).
In addition, an example of such a technique includes one by means of which capacitance is further improved by lowering an etching layer by applying pressure thereto. (Patent Document 3).
In recent years, digitalization of electronic information devices requires an electrolytic capacitor provided with this electrode foil to be smaller-sized and to have higher capacitance and lower impedance in a high frequency region. In particular, in the case of communication devices such as a personal computer or a cellular phone, a capacitor is strongly demanded having much higher capacitance in accordance with the higher operation speed of a CPU employed.
An example of a conventional and widely-used capacitor for a communication device includes a layered ceramic capacitor that meets the demand for miniaturization. However, this capacitor cannot meet the demand for higher capacitance. Hence, an electrolytic capacitor is in use as a capacitor having a low equivalent series resistance value (ESR value) and high capacitance, and allowing for miniaturization.
Besides, some attempts were made with respect to an electrode foil for electrolytic capacitor to achieve a further reduced ESR value by thickening the remaining core portion of an electrode foil, that is, an unetched portion thereof (Patent Documents 4 and 5).    Patent Document 1 Japanese Laid-open Patent Publication No. 2005-203529    Patent Document 2 Japanese Laid-open Patent Publication No. 2005-203530    Patent Document 3 Japanese Laid-open Patent Publication No. H10-189398    Patent Document 4 Japanese Laid-open Patent Publication No. 2003-59768    Patent Document 5 Japanese Laid-open Patent Publication No. 2003-59776
Meanwhile, electrolytic capacitor having such an electrode foil is for use in a vehicle. In vehicle use, the limited space for installation in a vehicle forms a restriction of the space for electronic parts to be employed therein. However, electronic control devices for in-vehicle use have increasing number of functions; an airbag, in particular, operates at an increasing number of places ranging from a cab seat, a passenger seat, a side and a curtain, whereby much higher capacitance is required for an electrolytic capacitor to be used as an energy source for operation of the air bag.
In spite of this, as stated above, the space for installing electrolytic capacitor is limited while the capacitor of the size similar to the conventional ones and having higher capacitance is required. Further, in order to supply energy for all those parts including a cab seat and a curtain, the required amount of capacitance is too high to be supported by an electrolytic foil produced by way of the conventional etching technology.